This invention relates to a process and an apparatus for separating polycyclic and polyhalogenated hydrocarbons, particularly polyhalogenated dibenzodioxins and dibenzofurans, from the exhaust gas of a sintering process, particularly a process of sintering iron ore.
Fine-grained or pulverized materials can be compacted by sintering to provide shapes, such as granules and pellets, which can more easily be handled. Sintering is used, e.g., to compact ores. For that purpose the fine-grained ore is mixed with fine-grained carbonaceous fuels, is agglomerated and is heated to temperatures above 1000.degree. C. in sintering plants supplied with oxygen-containing gases. The agglomerates of sintered ore consist of granules or pellets and have a certain mechanical strength so that they can readily be reduced to metals, e.g., in blast furnaces. Sinter roasting is used to roast fine-grained sulfide ores and to process them at the same time to form sintered granules and pellets.
Sintering involves a decomposition, particularly of carbonates and sulfates, an oxidation of sulfides, an evaporation of readily volatile oxides, sulfides, and chlorides, and an expulsion of bound water. As a result, the exhaust gases formed in the sintering process contain the gaseous reaction products CO and CO.sub.2 formed by the combustion of carbon and the gaseous pollutants which have been released from the ores.
Since halogenides, particularly chlorides, as well as carbon are present during the sintering process, polycyclic and polyhalogenated hydrocarbons and particularly polyhalogenated dibenzodioxins (PCDD) and polyhalogenated dibenzofurans (PCDF) are formed, which are contained in the exhaust gas stream leaving the sintering plant. Finally, the exhaust gas stream from the sintering process is contaminated with dust-like solid particles. The gaseous pollutants, particularly CO, SO.sub.2, HCl and HF, as well as the dust and the polycyclic and polyhalogenated hydrocarbons, particularly the polyhalogenated dibenzodioxins and dibenzofurans, must be separated from the exhaust gas before it is discharged into the atmosphere.
Published German Patent Application 41 05 214 discloses the purification of the exhaust gas stream from a sintering plant in a process in which the exhaust gas stream withdrawn from sintering conveyors is dedusted in an electrostatic precipitator plant, the exhaust gas stream leaving the electrostatic precipitator plant is moistened with a finely dispersed aqueous treating solution, which contains hydrogen peroxide, the exhaust gas stream is subsequently fed to a scrubber, which contains a corona electrode assembly and a succeeding scrubbing zone, wherein the exhaust gas stream which has been moistened and has been conducted through the corona electrode assembly in a dropless state is contacted in the scrubbing zone of the scrubber with a finely dispersed scrubbing liquid, which contains finely ground activated carbon or finely ground hearth furnace coke. In an alternative of the known process, the moistened exhaust gas stream which has been conducted through the corona electrode assembly in a dropless state is contacted in the scrubbing zone of the scrubber with a finely dispersed scrubbing liquid, which is conducted in a scrubbing liquid circuit and before entering the scrubber is passed through an activated carbon filter or a bed of granular hearth furnace coke.
Published German Patent Application 41 13 597 discloses for the purification of exhaust gas a process in which dust, HCl, HF, SO.sub.2 and optionally NO.sub.x are removed and pollutants, particularly dioxins and furans, are removed by adsorption by an adsorbent, which consists of zeolites having a SiO.sub.2 -to-Al.sub.2 O.sub.3 mole ratio of 2 to 6. In that process it is also contemplated that the exhaust gas is first mixed with the zeolites and is subsequently passed through a cloth filter, which contains a filter layer, and 30 to 700 mg zeolites having a particle diameter of 1 to 100 micrometers, particularly of 5 to 20 micrometers, are supplied to the exhaust gas per m.sup.3 thereof.
Published German Patent Application 40 12 982 discloses for the purification of gases and exhaust gases to remove inorganic and organic pollutants a process in which finely powdered activated alumina, silica gel, diatomite, finely powdered zeolites and/or similar inorganic substances are blown into the gas stream, gas and solids are intensely mixed, the solids are applied to a filter surface and are left on the filter as a renewable loose adsorbent layer having a sufficient depth, and the pollutant-laden solids are mechanically removed from there. In the known process it is contemplated to use the inorganic surfactants in the form of a finely powdered material having a particle size distribution in which 100% of the particles are less than 100 micrometers, preferably less than 50 micrometers. That process is to be used, inter alia, to separate dioxins and furans.
Published German Patent Application 41 28 106 describes the selective separation of highly condensed polycyclic hydrocarbons, particularly of halogenated dibenzodioxins and dibenzofurans, from previously dedusted exhaust gases, which contain SO.sub.2, H.sub.2 O, and heavy metals, in a process in which the hydrocarbons are adsorbed on solid adsorbents, which consist of a dealuminized zeolite, which has an SiO.sub.2 /Al.sub.2 O.sub.3 ratio from 20:1 to 1000:1; when the adsorption is effected at a temperature from 20.degree. to 200.degree. C.; the zeolite has a particle diameter from 1 to 5 mm and is contained in a fixed bed reactor or a moving-bed reactor.
German Patent Application P 44 03 244.7 discloses for the purification of oxygen-containing exhaust gases formed by the incineration of garbage, industrial waste materials, and sewage sludge a process in which polyhalogenated hydrocarbons, inter alia, are removed from the exhaust gases by adsorption on zeolites, the exhaust gases above their dew point are reacted at a temperature from 80.degree. to 180.degree. C. and a gas velocity from 3 to 20 meters per second with a mixture of naturally occurring zeolites in a gas-solids suspension for a reaction time from 0.5 to 10 seconds, wherein the median particle size of the mixed zeolites is from 5 to 50 micrometers and the mean suspension density of the gas-solids suspension is from 0.02 to 10 kg solids per sm.sup.3 of exhaust gas (sm.sup.3 =standard cubic meter of dry gas).
Finally, German Patent Application P 44 13 280.8 discloses the separation of polyhalogenated dibenzodioxins and dibenzofurans from the exhaust gas from a sintering process, particularly a process of sintering iron ore, in a process in which the entire dust-containing exhaust gas from the sintering conveyor is contacted above the dew point at a temperature from 90.degree. to 180.degree. C. and at a velocity from 6 to 20 meters per second with a mixture of naturally occurring zeolites in a gas-solids suspension for a reaction time from 0.5 to 10 seconds, wherein the median particle diameter d.sub.50 of the mixed zeolites is from 5 to 100 micrometers, the mean suspension density of the gas-solids suspension is from 5 to 4500 g solids per sm.sup.3 exhaust gas, the dust and the mixed zeolites are subsequently jointly separated from the gas-solids suspension, and part of the separated solids are again contacted with the exhaust gas from the sintering conveyor. That known process is carried out in an apparatus which consists of a solids-entraining reactor, a multifield electrostatic precipitator, and a solids recycle line, which connects the collecting space of the first field of the electrostatic precipitator or the collecting spaces of the first and second fields of the electrostatic precipitator to the exhaust line leading to the solids-entraining reactor. In that apparatus the electrostatic precipitator may be replaced by a bag filter.